Transfer across cellular membranes of inorganic ions and of organic nutrients by the human lens in vitro is to be investigated in order to learn whether there are selective losses of the capability to transport representative essential solutes with ageing or in cataract. Ligands include amino acids, monosaccharides, nucleosides, Rb+ and Ca++. When feasible, lenses from the same individual will be incubated to determine mediated influx of these substances. This is accomplished by measuring uptake of radioisotope with or without specific competitors or inhibitors of a given system of transport, or in the presence and absence of external Na+. Under appropriate conditions, the existence and activity of seven transport-systems for amino acids are to be evaluated. Integrity of the Na+, K+ activated ATPase is assessed by measuring uptake of Rb+ with or without ouabain, and the mediated component of Ca++ as the net flux with or without 20X molar excess of Sr++. Cellular uptake of the ligands is calculated in all experiments by utilizing an impermeable indicator ((3H)-D-mannitol or (14C)-sucrose) and applying the extracellular correction obtained therewith. The metabolic condition of the lens is routinely monitored by determination of formation of lactate and of the level of ATP. Lactate is assayed either by conversion to acetaldehyde followed by reaction with P-phenylphenol, or with lactic dehydrogenase-NAD. ATP is quantitated by bioluminescence with luciferin-luciferase. The few investigations which have been conducted on human lenses indicate that altered permeability to Na+, K+ and Ca++ occurs in cortical cataract. There is also limited or indirect evidence of impaired transport of amino acids in senile cataract. We intend to expand on this work with the objective of more clearly delineating the significance of transport across cellular barriers in the aging human lens.